Micro-electromechanical switching device

ABSTRACT

The invention relates to an electromechanical switching device including at least one pair of inductive elements electrically connected in series, said inductive elements being intended to generate two magnetic fields when current is flowing through said inductive elements, the interaction between these two fields resulting in a displacement of at least one of the inductive elements and a displacement of a mobile contact element linked to said at least one inductive element and intended to switch between two positions, at least one of these positions enabling an electrical connection between at least two conductive elements. The invention uses the mechanical forces exerted on at least one inductive element able to move thanks to two electro-magnetic fields oppositely generated by two inductive elements to activate a switch effect between two positions.

FIELD OF THE INVENTION

This document relates to a micro-electromechanical switching device andto a process for fabricating such a micro-electromechanical switchingdevice.

BACKGROUND OF THE INVENTION

Electromechanical relays are switching devices typically used to controlhigh power devices. Such relays generally comprise two primarycomponents: a movable conductive cantilever and an inductive element,generally an electromagnetic coil. When activated, the electromagneticcoil exerts a magnetic force on the beam in the same way that a magnetwill pick up a nail. This causes the beam to be pulled toward the coil,down onto an electrical contact, closing the relay by creating anelectrical connection. Said electrical connection may be galvanic ormore often based on a capacity variation. The more important thecapacity is, the more it will enable a current having a given frequencycrossing the switching device. These micro-electromechanical relays havebeen down-sized in order to fit the needs of modern electronic systems.The micro-electromechanical relays do not present limitations observedfor solid-state relays that require large and expensive heat sinks asresistances of such devices on ON and OFF position are generally oneorder of magnitude higher than for electromechanical switches and causea strong heating effect.

For example, the document U.S. Pat. No. 6,094,116 proposes an improvedmicro-electromagnetic switching device. The structure proposed in thisdocument allows a unique powerless hold feature. A magnetic layer isfirst deposited on the substrate. An electromagnetic coil is thencreated adjacent to this material. A deflectable structure in a magneticmaterial is then laid down in order to have a portion over or adjacentto at least one electrical contact. In operation, current passes throughthe coil, causing the deflectable structure to deflect, and either makeor break contact with the electrical contacts.

This implementation of an electromechanical switch offers a goodminiaturization but it requires the deposition of a magnetic materialand requires specific current or voltages to switch from one position tothe other.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose amicro-electromagnetic switching device having many advantages regardingthe state of the art and, especially not requiring the deposition of amagnetic material.

To this end, an electromechanical switching device according to theinvention includes at least one pair of inductive elements electricallyconnected in series, said inductive elements being intended to generatetwo magnetic fields when current is flowing through said inductiveelements, the interaction between these two fields resulting in adisplacement of at least one of the inductive elements and adisplacement of a mobile contact element linked to said at least oneinductive element and intended to switch between two positions, at leastone of these positions enabling an electrical connection between atleast two conductive elements.

The invention uses the mechanical forces exerted on at least oneinductive element able to move thanks to two electromagnetic fieldsdistinctly generated by two inductive elements to activate a switcheffect between two positions. Advantageously said two magnetic fieldsare opposite. The current in the inductive elements plays the role of acontrol line enabling the switch between two positions of the mobilecontact element. Consequently, the inductive elements of the switch cansimply be inserted on a supply line of a function. Said switch can thencontrol part of this same function or another function. No extra currentdedicated to the control of the switch is needed. Effectively, whateverthe sign of the current is, the switch will have the same behavior.Moreover, it has to be noted that this switch is well integrated andsmall.

In a specific embodiment, an insulation is provided on conductiveelements in order to calibrate given values of capacitors between saidcontact element and said conductive elements during the connection. Thevalue of the capacitor will then decrease significantly during theswitch to the position where no connection is realized. In this case,the switch is based on a variation of capacitance.

In a simple embodiment, the two inductive elements of a pair are indistinct and parallel planes and superimposed on each other. Aconductive link is provided between the two inductive elements in orderto connect them in series. Advantageously, the contact element isimplemented in one of these planes.

In a simple implementation of the invention, inductive elements areelectromagnetic coils coiled in opposite directions. According to theinvention, the central points of the coils advantageously link the twocoils of one pair. One of the coils and, consequently, the mobileelement attached to this coil, is free to move. The separation betweenthe two coils of one pair is advantageously realized by under-etching anoxide layered between the two coils according to a process of theinvention presented in the following.

In such an implementation, the coils are used as DC inductors as theygenerate magnetic fields, as guiding elements as they guide the movementof the mobile element, springs as their return force helps in theestablishment of the non-activated position when no current is flowinginto the coils and blocking coils in RF as they are cutting highfrequencies that could cause noise in the circuit linked to the switch.Consequently, the invention helps at having a very good behavior for aswitch as it provides other advantageous functions by itself.

In a preferred embodiment, a second pair of inductive elements isconnected to the first pair by connection of one of the inductiveelements of the second pair to the contact element.

It will be demonstrated hereinafter that, for example, the use of fourcoils is the simplest way to realize a return of the current on theplane of the fixed inductive elements.

In an advantageous embodiment, said switching device is placed in acavity. For example, this cavity is realized by flip-chip technologies.According to an alternative of the invention, said cavity is providedwith an electrode intended to enter into contact with said contactelement. This alternative allows having two positions that do notconsume any power. Effectively, an impulsive current is only necessaryto make the mobile element stick to the electrode. This current impulsedoes not require any power consumption and keeping the mobile elementstuck to the electrode does not require any power, a voltage beingsufficient.

The invention finds its application in any circuit where a switch isadvantageously provided. Especially the switch according to theinvention can be used in a circuit where a function of reception isactivated by a current, the switch according to the invention beingplaced between this function and the element from which the supplycurrent for this function is generated, said switch being intended tocontrol part of this function or another function.

The invention also relates to a method to fabricate an electromechanicalswitch according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereafter in detail with reference to thediagrammatic Figures wherein:

FIG. 1 shows a perspective schematic view of a first alternative of theinvention;

FIG. 2 shows an electrical assembly according to the first alternativeof the invention, this electrical assembly being an illustration of acircuit according to the invention;

FIG. 3 shows a perspective schematic view of a second alternative of theinvention;

FIG. 4 shows an electrical assembly according to the second alternativeof the invention, this electrical assembly being an illustration of acircuit according to the invention;

FIG. 5 shows a perspective schematic view of a second alternative of theinvention;

FIG. 6 is a block diagram of a circuit according to a preferredapplication of the invention;

FIG. 7 is a schematic diagram of a telecommunication apparatus whereinthe invention is advantageously implemented.

DESCRIPTION OF EMBODIMENTS

The micro-switching device of the present invention is fabricated by aprocess that is based upon technologies ordinarily used by integratedcircuit manufacturers and eliminates the need for expensive deviceassembly. A process utilizing classical micro-electronic andmicro-machining technologies will be described below.

Referring to FIG. 1, a micro-electromechanical switch according to theinvention comprises two pairs of inductive elements C1 a, C2 a and C1 b,C2 b. This Figure is representative of a preferred embodiment of theinvention but is not described in exclusion of other ways to realize theinvention. Each of these pairs includes a first inductive element C1,for example an electromagnetic coil, in a first plane and a secondinductive element C2, for example an electromagnetic coil in a secondplane parallel to the first one. Said second inductive element C2 a (orC2 b) is superposed on the first corresponding inductive element C1 a(orC1 b), connected with it by a conductive via VIa (or VIb). Said secondinductive element C2 is fabricated in order to produce a magnetic fieldopposite to the one created by the first inductive element C1 as soon asa current is flowing through these inductive elements. The same currentis flowing through the four inductive elements as they are connected inseries. One of the two inductive elements of each pair is mobilerelative to the other. In the case described in FIG. 1, C2 is mobilerelative to C1. Advantageously, the inductive elements areelectromagnetic coils, as represented on FIG. 1. In such an advantageousand simple implementation, the corresponding coils are simply coiled inopposite directions in order to produce opposite magnetic field as soonas a current is flowing through said coils. The switch represented bythe four coils presents an input connection CIN and an output connectionCOUT in order that a current can be provided to the switch. Such acurrent will control the switching. The switch will be called activatedwhen a current flows in the inductive elements and non-activated when nocurrent flows.

According to the preferred embodiment of the invention, as presented inFIG. 1, when current is flowing in coils coiled in opposite ways, thesecond coils C2 will lift by the electromagnetic force. According to theinvention a contact element CEL is for example attached to the twosecond coils C2. This contact element CEL is mobile as well as thesecond coils and will lift with the second coils C2 causing a firstposition of the switch.

When no current is flowing in the coils, the mobile element is generallypart of an RF capacitor, for example polarized in DC, so that anelectrostatic force will stick the mobile contact element CEL toconductive elements CCT for example realized on the plane of the firstcoil. This causes a second position of the switch. The polarization ofsaid capacity may be optional as the natural adhesion of materials maybe sufficient to maintain the contact element CEL close to conductiveelements CCT.

Said contact element CEL is then intended to switch between twopositions, called first position, here corresponding to the activatedswitch, and second position, here corresponding to the non-activatedswitch. These two positions are not represented in FIG. 1 for reasons ofclarity of the drawing. Nevertheless, FIG. 2 helps understanding the twopositions by representing this time the contact element CEL in thesecond position in a full line compared to the first position that isrepresented in FIG. 1 and represented in FIG. 2 by a dotted line.

In said first position, so when the switch is activated as representedin FIG. 1, the contact element CEL is far from conductive elements CCTprovided on the first plane, a weak capacitance being observed betweenthe contact element CEL and the conductive elements CCT. Effectively,the contact element is part of the RF capacitor and the value of thecapacitor decreases significantly when the switch is activated. In thepreferred embodiment represented in FIGS. 1 and 2, when the switch isactivated, the switch does not provide a connection path between theconductive elements CCT.

In said second position, the contact element is close to conductiveelements CCT provided on the first plane. This second position of thecontact element CEL generates a connection path between the conductiveelements CCT. This connection path may be for example galvanic or basedon a variation of capacitance.

In case of a switch intended to enable a galvanic contact between theconductive elements CCT, said mobile contact element CEL or a part ofthe mobile element CEL or an element linked with the mobile contactelement CEL comes into galvanic contact with the conductive elementsCCT. In this case, in order to have good contacts, special materialsshould constitute the conductive elements and the mobile element (or thepart of it or the element linked to it): gold, platinum. In this case,advantageously, part of the mobile element is intended to serve in acapacitor for maintaining the mobile contact element CEL in the secondposition by the electrostatic force and part of the mobile contactelement CEL is properly dedicated to serve for the galvanic contacts.

In case of a switch based on a variation of capacitance, the connectionpath comprises the formation of two capacitors in series. In secondposition, the values of the capacitors are higher than in the firstposition, the values of the capacitors decreasing significantly when theswitch is activated. Said capacitors enable a current of a givenfrequency to go through the switch from one conductive element CCT tothe other conductive element CCT, said current being reproduced from onecapacitor to the other by the common electrode constituted by the mobilecontact element CEL. In a specific embodiment, insulation is provided onconductive elements CCT in order to calibrate the values of capacitorsbetween said contact element CEL and said conductive elements CCT.Maintaining the contact element CEL and connection path is thenadvantageously realized by the same contact element CEL.

It has also to be underlined that in the preferred embodimentrepresented in FIG. 1, the two pairs of coils help the mechanicalguidance of the displacement of the different mobile part of a switchaccording to the invention. They have also the role of springs and exerta kind of return force that goes to the direction of the electrostaticforce that will stick the mobile contact element CEL onto the conductiveelements CCT. This electrostatic force is advantageously generated bythe polarization of the capacitor in DC as described in FIG. 2representing an electrical assembly of a switch according to theinvention.

Referring to FIG. 2, a possibility of assembly for the preferredembodiment of the invention is presented. In this figure are representedthe four different coils connected in series C1 a, C2 a, C2 b and C1 b.The two coils of a pair are linked by conductive vias VIa and VIb asrepresented above. The contact element CEL is linked to a point situatedbetween C2 a and C2 b as represented physically in FIG. 1. This contactelement CEL is moving between two positions: a first position in adotted line and a second position in a full line. The switching betweenthese two positions is realized by the action of different forces. Theelectromagnetic force FEM generated by the superposed coils makes thecontact element CEL and the second coils C2 lift. An electrostatic forceFES makes the contact element CEL contact conductive elements CCTlayered on the first plane. This electrostatic force FES is generated bythe fact that the capacitor, materialized by the contact element CEL andthe conductive elements CCT on the first plane, is for examplepolarized, by voltage VCC.

A functional circuit RFF is linked to the switch according to theinvention. As a simple current flowing through the coils is necessary toactivate the switch, the latter can be placed simply in series with asupply current line of this functional circuit RFF. In this case, noextra current is required for activation of the switch. This is animportant advantage of the invention. As soon as the functioning of thefunctional circuit RFF is required, the supply current Ic of thefunctional circuit RFF flows in the coils and activates the switch. Thefunctioning of the functional circuit RFF can be independently launchedby known means: a control link or serial bus. VBAT is the voltage thatis supplied to the functional circuit. Such a functional circuit can beany consumer electronic circuit realizing a specific electronicfunction. For example, this functional circuit RFF is a circuit managingthe transmission protocols that control power amplifier functions(active during transmission) and reception functions (active duringreception). Variable currents absorbed by these functions can then beused to control the coils and activate the switch. Such a functionalcircuit is for example implemented in a telecom terminal where twooperating modes are used: transmission and reception. Then the inventionalso relates to a circuit including a micro-electromechanical switchingdevice as described above for implementing a switch between two types ofbehavior of said circuit. Said circuit includes functional circuits orfunctional parts that can be activated or deactivated using the switch.FIG. 2 gives a schematic representation of such a circuit.

In a particular application, the invention may advantageously beimplemented in a circuit FCS as represented in FIG. 6. This circuitincludes a reception chain for received signals RX and a transmissionchain for transmitted signals TX with a commutation device COM linked toa line common for reception and transmission, for example an antennaANT. Reception and transmission chains each include at least a filter,FIR and FIT respectively, which is linked to an amplifier, RA and TArespectively. Commutation device COM is advantageously realized withswitching devices according to the invention and implemented asexplained above. According to the preferred embodiment of the invention,when the switch is activated, no connection path is provided.Consequently, a switch according to the preferred embodiment of theinvention can be advantageously implemented to close a contact for thefunctioning of a reception function when the transmission function isnot activated and consequently no supply current is provided to thistransmission function. Another switch according to the preferredembodiment of the invention can be implemented to do the opposite task:when activated by the supply current of a reception function open aconnection path for a transmission function. Many kinds of commutationdevices can then be realized using a switching device or severalswitching devices of the invention in combination. In the following arealso represented switching devices that enable a connection path to beestablished while the switch is activated.

A circuit FCS as represented in FIG. 6 is advantageously used in aelectronic telecommunication apparatus as represented in FIG. 7 andintended to receive and transmit signals. This telecommunicationapparatus advantageously implements a circuit FCS as describedhereinabove. Moreover it includes at least an antenna ANT, amplifiers RAand TA and processing means to process signals MC.

The preferred embodiment of the invention has been described but variousother embodiments based on the principle of the invention are includedin the scope of the invention. Several examples will follow to show thediversity of possibilities offered by the principle of the inventiondefined by the claims. These examples present among other things thepossibility to use a single pair of inductive elements, the possibilityto have an activated switch generating a connection path (as opposed tothe preferred embodiment), the possibility to have two powerlesspositions of the switch.

To protect the switch as described hereinabove it may be useful to putit in a closed cavity. This cavity is also advantageously hermetic. Thecavity can be realized, for example, by flip-chip technologies.

According to a basic embodiment of the invention, only one pair ofinductive elements is realized. In this case, the current flowingthrough the first and second inductive element has to be returned on anon-mobile plane. Consequently, at least a flexible conductive via,enabling the second coil to be deformed, has to be provided. Quite animportant deformation is required for such a conductive via that has tobe quite a long one in zigzag or in spiral. Such a conductive via takesplace in the integrated circuit. Consequently, it is highlyadvantageous, according to the preferred embodiment, to use two pairs ofspirals as inductive elements as the place is taken in any case.Moreover, spirals allow having a long link on a very small surface.Nevertheless, the invention can be implemented with a single pair ofinductive elements: a specific example will be given hereinafter.

An advantageous embodiment of a simple implementation using a singlepair of coils in a cavity is represented in FIG. 5. A simple conductiveflexible via VIF is provided to enable the displacement of the secondcoil C2 and the circulation of the current in both coils between the twoconnection pads CIN and COUT. The conductive via could also benon-flexible, the spires of the coil C2 serving as flexible part, onlysome central spires of C2 being displaced according to the invention.This displacement is then generally observed to be more important forthe internal spires than for the external one as the external one ismore or less constrained by the presence of the conductive via VIF. Inthis case the mobile contact element CEL can be one of the internalspire of the coil C2 and conductive elements CCT are provided on top ofthe cavity. The cavity is not represented for reasons of clarity. Theconnection is realized by displacement of the spires of the coil C2according to the principle of the invention. In this case, the positionwhere a connection path is realized corresponds to the one where theswitch is activated by a current flowing in the coils C1 and C2, thesecond coil C2 being consequently in a “high” position. Here theactivation of the switch enables a position where a connection path isestablished opposite to the behavior of the switch of the preferredembodiment.

Referring to FIGS. 3 and 4, an alternative of the invention comprisesadding an electrode EL to the side of the cavity opposite to theconductive elements CCT layered on the first plane. This electrode EL isfor example linked to a voltage generator VOL. No current flows in thiselectrode, so no power is consumed. Nevertheless, the voltage generatorVOL allows a second electrostatic force FESM to make the mobile contactelement CEL stick on this electrode EL. The voltage generated can be,for example, of the order of one to ten volts. This voltage generatorVOL can be activated as soon as, for example a current is flowing in thecoils. The advantage is that the contact element CEL can be kept in the“high” position without any circulation of current in the coils. To makethe contact element CEL return to the “low” position (where a connectionpath is realized), the voltage has simply to be put to zero. The returnforce generated by the coils that constitute springs, helps this return.In this alternative of the invention, the switch has two stablepositions that do not necessitate any power consumption. Only an energyimpulse realized by a current impulse in inductive elements is necessaryto make the contact element CEL change its position by electromagneticforce.

The invention also relates to a process to fabricate a switch or relayintended to switch between two positions, at least one of thesepositions enabling an electrical connection between at least twoconductive elements. Such a process uses techniques conventionally usedin integrated circuitry. First, at least one inductive element isformed. Several possibilities using classical microelectronic processexist to form such an inductive element. For example a layer ofconductive material is deposited. A mask then allows etching theconductive material in order to form the inductive element, for examplea coil. The conductive material is generally a metal as for example,aluminum. It is also possible to form a mold structure defining at leastone location for at least one electromagnetic coil. Etching a substrateusing a mask can form such a mold structure. This mold structure is forexample realized in a high impedance substrate to have a good insulationof the RF contacts. Within the mold structure is deposited a conductivematerial, generally a first metal, in sufficient quantity to build up atleast one electromagnetic coil.

Then, an under-etchable material is deposited above said inductiveelement. A conductive link is arranged through the under-etchablematerial to then connect the two inductive elements. The under-etchablematerial is, for example, oxide.

Advantageously an insulating material is deposited between the firstinductive element and the under-etchable material. This insulatingmaterial is not under-etchable and constitutes a kind of protectivelayer on the inductive element. Such a protective layer can, forexample, be constituted by nitride. For example, 0.4 μm of nitride and 1μm of oxide are deposited.

At least one second inductive element is formed above saidunder-etchable material. The under-etchable material is thenunder-etched. For example a layer of conductive material is deposited. Amask then allows to etch the inductive element, for example a coil.

The conductive material is generally a metal as for example, aluminum.The under-etchable material is then under-etched in order to free thesecond coil. Simple via interconnecting metal layers realize contactsbetween the two coils of a pair. The two first coils in the first planeand second coils in the second plane can be realized in different metalsor in the same metal. Insulating material can be layered to calibratethe values of capacitors causing the connection path to form. As seenabove the conductive elements to form a connection path in the switchaccording to the invention can be implemented on the first plane in thesame processing step as the formation of the first coil or on top of acavity. Those conductive elements can have any position regarding aswitch of the invention as soon as the contact element can form aconnection path by moving towards said conductive elements.

An example of implementation is proposed according to the preferredembodiment of the invention with two pairs of concentric coils in twodistinct planes. These coils have 7 spires. The first one is for exampleconstituted by aluminum and is 1 μm thick and 6 μm large. The second oneis for example constituted by aluminum and is 3 μm thick and 5 μm large.As an example, a current of 60 mA flowing in the coils generatesdisplacement of 20 to 50 μm of the coils. According to the differentgeometry, the values of the capacities assuring the RF switch functionare around 0.1 to 1 pF and will decrease when the contact element is farfrom the conductive elements that realize the contact. This example isnot restrictive and many other dimensions and physical characteristicscan be changed without being excluded from the scope of the invention.Any form of inductive element different from a coil can also be used inthe invention. Nevertheless, the advantage of coils is that they behaveas blocking coils in RF as they cut the high frequency signals that cangenerate parasitic ways. They behave effectively as self-inductances athigh frequencies.

1. A micro-electromechanical switching device including at least onepair of inductive elements electrically connected in series, saidinductive elements being intended to generate two magnetic fields whencurrent is flowing through said inductive elements, the interactionbetween these two fields resulting in a displacement of at least one ofthe inductive elements and a displacement of a mobile contact elementlinked to said at least one inductive element and intended to switchbetween two positions, at least one of said positions enabling anelectrical connection between at least two conductive elements.
 2. Amicro-electromechanical switching device according to claim 1, whereinsaid two magnetic fields are opposite.
 3. A micro-electromechanicalswitching device according to claim 1, wherein an insulation is providedon conductive elements in order to calibrate values of capacitiesbetween said contact element and said conductive elements.
 4. Amicro-electromechanical switching device according to claim 1, whereinsaid inductive elements are in two distinct and parallel planes andsuperimposed on each other.
 5. A micro-electromechanical switchingdevice according to claim 1, wherein inductive elements areelectromagnetic coils coiled in opposite ways.
 6. Amicro-electromechanical switching device according to claim 1, wherein asecond pair of inductive elements is connected to the first pair byconnection of one of the inductive elements of the second pair to thecontact element.
 7. A micro-electromechanical switching device accordingto claim 1, wherein said device is placed in a cavity, said cavity beingprovided with an electrode intended to enter in contact with saidcontact element.
 8. A circuit including at least onemicro-electromechanical switching device as claimed in claim 1, forcausing a commutation to occur between two operating modes of at least afunctional part of said circuit.
 9. A telecommunication electronicapparatus including at least an antenna, at least an amplifier,processing means to process signals, said processing means comprising atleast a circuit as claimed in claim
 7. 10. A method for manufacturing amicro-electromechanical switching device intended to switch between twopositions, at least one of said positions enabling an electricalconnection between at least two conductive elements, by the steps of:forming at least one first inductive element on a substrate, depositingan under-etchable material above said inductive element, forming atleast one second inductive element above said under-etchable material, aconductive link being arranged through this under-etchable material toconnect the two inductive elements, forming a contact element linked tosaid second inductive element above said under-etchable material,under-etching the under-etchable material.